The research proposed herein utilizes a novel assay for proteins from eukaryotic cells that can catalyze the initiation of the synthesis of DNA chains on single-stranded regions of template DNA. Such proteins will be isolated from yeast, mouse, and human cells. Their mechanisms of action and specificities for DNA templates will be examined to define the different pathways for the initiation of the synthesis of DNA chains. The activities and specificities of these proteins will be compared in normal and virus-infected cells. Different viruses which stimulate or inhibit host DNA synthesis will be used, as well as those which transform host cells. Our goal is to understand the biochemistry of the initiation of DNA synthesis in eukaryotes, and the interaction of the virus chromosome with this part of the host DNA replication systems.